Surface total solar radiation variability at Athens, Greece since 1954

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1 Surface total solar radiation variability at Athens, Greece since 1954 S. Kazadzis 1, D. Founda 1, B. Psiloglou 1, H.D. Kambezidis 1, F. Pierros 1, C. Meleti 2, N. Mihalopoulos 1 1 Institute for Environmental Research and Sustainable Development (IERSD), National Observatory of Athens (NOA), Metaxa & Vas. Pavlou, Penteli, 15236, Athens, Greece. 2 Physics Department, Aristotle University of Thessaloniki, Greece *corresponding author kazadzis@noa.gr Abstract Long-term time-series of Surface Solar Radiation (SSR) have shown significant temporal variation of the incoming solar radiation intensity for different locations worldwide. Measurements of surface SSR prior to 80 s have shown a global decrease; attributed to changes in the transparency of the atmosphere and more specifically to alterations in cloudiness, atmospheric aerosols and gases interacting with incoming solar radiation. On the other hand, a large number of studies based on surface SSR measurements after the 80 s have shown a reversal of this trend. Finally, solar networks operating after 2000 have shown mixed tendencies. The National Observatory of Athens (NOA) has been conducting SSR measurements at the historical station of Thiseion (Athens center) intermittently since December Results from an analysis of NOA s SSR time-series show short-term variability related to cloud-cover variations. In addition, a decrease is found in the annual SSR intensity in the order of 2.1% for the period and an increase of 4.4% from 1983 till today. 1 Introduction In the past decades solar radiation and the transmission of the atmosphere have been of increasing interest because of the related impacts on climate. Solar variations have been investigated in several studies using ground based total surface solar radiation (SSR) from various monitoring networks (Ohmura, 2009) worldwide and also by satellite derived estimations (Kambezidis et al., 2010). Recent studies (Wild, 2009 and references therein) have reported a worldwide decrease of solar incoming radiation for the period 1960 to 1980 (known as global dimming) followed by an increase (global brightening) from 1980 since today. The changes in cloud coverage and atmospheric transmission are the factors that are investigated in order to determine the possible causes of these changes. The effect of clouds and aerosols on the solar radiation variations over the past 50 years have been investigated by Ohmura, Using the 20-year dimming phase

2 from 1960 to 1980 and the 15-year brightening phase from 1990 to 2005, it was found that the aerosol direct and indirect effects played about an equal weight in changing global solar radiation. Concerning Europe, Ruckstuhl et al., 2008 suggested that the brightening phase occured under cloud free conditions and it is related with decreasing aerosol emissions (Streets et al., 2006). Weather systems are the main reason for changes in the radiation levels of the order of days. On the contrary, the presence of pollutants in the atmosphere through the processes of absorption and scattering could cause more systematic effects. The gaseous and particulate air pollutants may reduce solar radiation by up to 40% during air pollution episodes in polluted areas (Jauregui and Luyando, 1999). This attenuation is much larger during biomass burning events and volcanic eruptions. Long term series of SSR measurements are essential for such studies. Such measurement series have been recorded from the past 60 years at the center of Athens and are presented in this work. 2 2 Data and Methodology The SSR data used in this study cover the period from December 1953 to December 2012 and were measured by a series of pyranometers mentioned in table 1. The instruments operated continuously at the National Observatory of Athens (Lofos Nymfon, Thiseio), that is located at the center of Athens, Greece (38.0N, 23.7E, 107 m a.s.l.). Apart of the instruments and the period of operation, table 1 presents the maximum error on the calculation of daily and monthly values. The instrument spectral response is from nm and since 1986 it is a first class station. Table 1. Information on instruments operated at NOA since 1953 Instrument Period Class Maximum error (daily integral) Reference 1 Solarigraph GOREZYNSKI nd 5% Coulson (1975) 2 Eppley 180 pyranometer nd 5% Coulson (1975), Drummond (1965) 3 Eppley pyranometer, type Eppley Precision Spectral Pyranometer nd 3-5% Hulstrom (1989) st 1-2% Hulstrom (1989) Complementary to this study, cloud cover observations from the National weather service from 1954 have been used. The observations have been recorded at a site 7.5

3 Km away from the Observatory. Cloud observations were recorded in octas of cloud sky coverage. SSR data are processed using a set of quality control tests in order to ensure the quality of the data set. Post correcting procedure included: the rejection from the analysis of measuring days with more than 3 hours of missing data or two or more hours of consecutive missing measurements. Hourly missing values have been estimated by fitting the data on a two hour window. Mean daily insolation has been calculated from SSR (Joules/m 2 ). Months with more than 20 days of measurements have been used in the statistical analysis. Over the 59 years of measurements, only three months did not fulfill this criterion. 3 3 Results Mean daily insolation have been calculated using the complete data set. The annual pattern of mean daily insolation over the whole period, along with the 1-σ standard deviation is presented in figure 1. Fig. 1. Annual variability of insolation (green) along with the 1 σ standard deviation After calculating the mean daily insolation for each month we have calculated the de-seasonalized monthly values. Most of the cases are well within the ±10% from the normal while a few are exceeding ±20%. Τhe yearly de-seasonalized means from mean daily measurements have been calculated and presented in figure 2. In addition, mean monthly cloud coverage in octas has been calculated. Cloud octas averages for observations during the day have been used. In addition, each value has been weighted by a factor depending on the average SSR of the particular time compared with the daily SSR sum.

4 4 Fig. 2. De-seasonalized monthly (dash line) and yearly insolation (blue line). Also, de-seasonalized cloud coverage is presented in green. The correlation of yearly insolation and the mean yearly cloud coverage is presented in the superimposed figure. Τhe whole period was divided in two 30 year periods ( ) and ( ) in order to determine possible changes over the two periods based on a climatology of 30 years of SSR measurements for each one. In order to detect possible seasonal dependence on the results, the differences for each particular month were calculated. Using two separate 30 year climatologies we have found a total change in the order of 4.4% comparing the two periods. In addition, linear trends for the separate 30 year periods have been calculated and showed a total decrease of 2.2% for the first period followed by an increase of 4.5% for the second, while for the whole series an increase of 6% has been calculated. Signs of the Pinatubo eruption could possibly observed in the early 1990 s. It has to be clarified that linear trends on any period have been calculated using different monthly normal values which are consistent with each period. 4 Conclusions SSR for Athens area using a unique dataset covering a period of 59 years is presented. Results from the first 30-year period include uncertainties related with the calibration of the measurements, however the categorization of the station as second class provides the opportunity for useful information of solar radiation during that period. De-seasonalized monthly calculations of SSR and comparison with mean cloud coverage can be used in order to detect a SSR negative change till the start of the 70 s that can be partly attributed to enhanced cloud presence. Then a positive change

5 till 2012, is observed, with hints of larger rates on late 70 s-early 80 s and for the past decade. 5 Fig. 3. Left: differences between the two 30-year periods. Green line represents the differences for each month. Right: De-seasonalized monthly means of SSR for the two periods. Finally, we have tried to investigate possible changes on different periods for the whole 59 year data set. In order to accomplish that we have calculated linear trends over various time scales presented in figure 4 a and b. Month JAN APR JUL OCT All Months Number of years for estimated changes Fig. 4. Left: calculation of changes in % per decade for different timescales used (15 to 30 years). Each point on the line represents the middle year of the period used for the trend calculation Right: % change per decade for different timescales and for different months using 1954 as the starting year. (last 30 year period is magnified with changes presented as % per decade)

6 The quite evident anti-correlation of the independent sets of SSR and cloudiness provide useful information on explaining a large portion of year to year changes over the period. Comparing the 30-year periods ( and ) we have found a difference of 4.4% that shows no predominant link with seasonal features except for a possible drop in the difference during the last three months of the year. Following the month to month analysis presented in figure 3. Comparing this set of measurements with the results of various studies showing the dimming and brightening effects of the certain periods. We have observed a similar pattern (-2.1 and +4.5% respectively) with a lower magnitude. Analysis of changes of SSR for different time scales revealed that negative changes are observed till the seventies followed by positive ones in the order of 2-3% till today, excluding the possible effect of the Pinatubo eruption and the years that followed (negative and positive change in the start of 90 s). The decadal variations of SSR measured since 1954 at Athens, Greece originate from the alterations in the atmosphere s transparency, (namely clouds, aerosols and atmospheric pollution). The changes that has been presented include the cloud variability over the period, the increased anthropogenic pollution of the area till the mid 80 s and the decrease afterwards. The SSR database presented here, provides a unique opportunity of investigating and separating such effects over different periods. 6 References Coulson, K.L., Solar and Terrestrial Radiation: Methods and Measurements, Academic Press, Drumond, A.J., Techniques for the measurement of solar and terrestrial radiation fluxes in plant biological research: A review with special reference to arid zones, Proc. Montpiller Symp., UNESCO, Hulstrom, R. L., Solar resources, is the volume 2 in the Series: Solar Heat Technologies: Fundamentals and Applications, edited by Charles A. Bankston, The MIT Press, Cambridge, Ruckstuhl C, Philipona R, Behrens K, Collaud Coen M, Durr B, Heimo. A, Matzler C, Nyeki S, Ohmura A, Vuilleumier L, Weller M, Wehrli C, Zelenka A Aerosol and cloud effects on solar brightening and the recent rapid warming. Geophysical Research Letters 35:L DOI: /2008gl Wild M Global dimming and brightening: a review. Journal ofgeophysical Research 114. DOI: /2008JD Ohmura, A. (2009), Observed decadal variations in surface solar radiation and their causes, J. Geophys. Res., 114, D00D05, doi: /2008jd Kambezidis, Harry; Demetriou, Dora; Kaskaoutis, Dimitris; Nastos, Panagiotis Solar dimming/brightening in the Mediterranean EGU General Assembly 2010, held 2-7 May, 2010 in Vienna, Austria, p.10023